The invention relates to a die for forging a toothed portion of a toothed rack of a steering device, comprising first and second die parts, of which the first die part has a first molding recess for molding the toothing of the toothed rack, and the second die part has a second molding recess, which has the form of the back region of the toothed rack, lying opposite the toothing, and which can be moved together in a closing direction from an open position into an end position, in which they are at a distance from one another, while shaping a blank placed into the die, wherein, in the region of the molding recesses of the die parts, there is formed between the die parts a main cavity, which in the moved-together end position of the die parts is open on opposite sides toward secondary cavities, which respectively lie in a region lying between the first and second die parts. The invention also relates to a forging method for forging a toothed portion of a toothed rack for a steering device, wherein a blank is shaped between two die parts, of which the first die part has a first molding recess for molding the toothing of the toothed rack, and the second die part has a second molding recess, which has the form of the back region of the toothed rack, lying opposite the toothing, and which can be moved together in a closing direction from an open position into an end position, in which they are at a distance from one another, while shaping the blank placed into the die, wherein, in the region of the molding recesses of the die parts, there is formed between the die parts a main cavity, which in the moved-together end position of the die parts is open on opposite sides toward secondary cavities, which respectively lie in a region lying between the first and second die parts, and wherein, when the die parts move together, material of the blank is displaced into the secondary cavities.
Toothed racks for steering devices of motor vehicles that have constant toothings are often produced by machining, allowing a high degree of precision to be achieved. Such toothed racks may also be produced with sufficient accuracy by shaping. Shaping methods are often more cost-effective than machining methods. Toothed steering racks with variable toothing, in which the spacing of the teeth and/or the form of the teeth and/or the inclined position of the teeth changes over the extent of the toothing, are very difficult to produce. For cost-effective mass production, the methods of production become more complex.
Toothed steering racks which have a triangular cross section or a Y cross section in the region of their toothed ends are known. For example, such a toothed rack is disclosed by EP 0 738 191 B1 and the production of this toothed rack is performed by hot forging. Such toothed racks are well suited for variable toothings and are supported in their guidance against the influence of rolling moments that occur due to contact forces between the teeth of the pinion and the teeth of the toothed rack as a result of inclined positions by their longitudinal guidance. Toothed steering racks with a round back profile or D cross section also exist. Such toothed racks have some advantages over toothed racks with a Y cross section in production, including in their non-toothed regions, and in the fitting and sealing of the toothed rack. The required installation space is also significantly smaller and the geometry of the thrust piece mounting the toothed rack is simpler. However, these toothed racks are more susceptible to the influence of rolling moments, which can cause canting and associated noise (rack roll). For producing toothed racks, in particular round-back toothed racks, for steering devices by shaping techniques, forging methods with flash and forging methods without flash are known.
In the case of a forging method with flash, in which a die of the type mentioned at the beginning is used, when the two die parts are moved together, material of the blank is forced out of the main cavity into secondary cavities lying on both sides of the main cavity in the region of the parting plane between the die parts and this material forms flash, which is removed after the forging operation. This flash also makes up for volume tolerances of the blank. When the die parts are moved together, the opening of the main cavity toward the secondary cavities is continuously reduced (this is also referred to as the “die gap”), the internal die pressure increasing and parts of the material flowing from the main cavity into the secondary cavities. Particularly at the edges, there is a strong flow of the material of the blank under high pressure. This leads to great wear on the die parts, in particular at the edges of the die parts that bound the main cavity toward the secondary cavities. The die parts therefore have short service lives. Furthermore, the achievable toothing accuracies are limited. These also depend on the die gap geometry, which can be changed only by reworking the die parts themselves.
EP 1 007 243 B1 discloses a forging method with flash for producing round-back toothed racks with flash, the secondary cavities bounding the flow of the material of the blank from the main cavity. The total volume of the secondary cavities in the end position of the die parts corresponds in this case to the difference in volume between the toothed rack blank and the finished toothed rack in the toothed region. Consequently, in the end position of the die parts, the secondary cavities are closed and completely filled with material of the blank. As a result, an increased hydrostatic final pressure can be formed. One disadvantage of this method is that the volume of the blank has to be defined very accurately, and so it must be exactly pre-ground, or produced in some other way. This increases the complexity of the production considerably.
WO 2005/053875 A1 discloses a forging method without flash for producing round-back toothed racks. Two punches are provided between the two die parts. In the closed state of the two die parts, these parts abut against the two punches on both sides. At this point in time, the main cavity is not yet completely filled with the material of the blank. Thereafter, the two punches are forced into the main cavity, reducing the volume of the main cavity, whereby the material of the blank is pressed on all sides against the walls of the main cavity. The die that is used in this method, and is not of the generic type concerned here, consequently does not have any secondary cavities. Also in the case of this method, the volume of the blank must be accurately defined. Therefore, in the region of the toothed rack in which the toothing is formed there is formed a preform, which is reduced by the proportion of the volume that would occur in the case of production by machining. It has also been found that, in order to obtain a pleasing result, a geometrical shaping of the preform that approximates to the final form is generally required. The geometry of the preform must be empirically determined here, which is technologically complex. Moreover, the production of the preform causes significant additional costs, due to the machining itself and due to the requirements applying to the volume accuracy. Furthermore, the process control is complex and even minor deviations in the volume of the preform or in the volume of the main cavity may lead to the formation of flash, thereby generating further additional costs as a result of the reworking required.